(CBSNews.com) - An "unidentified substance inconsistent with oil" is emitting from several areas of BP's Deepwater Horizon rig wreckage, but no sources of leaking oil were identified. That's according to the Coast Guard, which oversaw BP's recent week-long mission to inspect the undersea wells and wreckage from the 2010 explosion.

The exact content of the leaking substance and how much is coming out is one mystery. But if it's not oil, then it means the source of recurring oil sheens that have recently been spotted around the Deepwater Horizon site remains unknown.

"No apparent source of the surface sheen has been discovered by this effort," said the Coast Guard's Capt. Duke Walker in a press release this afternoon. The Coast Guard did not say whether there are other parts of the Deepwater Horizon wreckage yet to be examined for leaks.

BP's remotely operated vehicle collected samples of the mystery leaking substance for further lab analysis, and the surface oil sheen is being monitored by satellite surveillance.

"Next steps are being considered as we await the lab results of the surface and subsurface samples and more detailed analysis of the video shot during the mission," said Walker.

The Coast Guard said BP's main Macondo well was observed during the subsea operation and found to be secure. Two relief wells, the riser pipe and the previously leaking containment dome were also to be re-examined, but the press release made no mention of them and the Coast Guard declined to answer further questions.

The Coast Guard says video of the inspections will be posted at the government Web site,www.restorethegulf.gov. The Olympic Triton ship launched and controlled the remotely operated vehicles during the operation.